1. Field of the Invention
The present invention relates to methods of determining lithological characteristics of an underground formation and, more particularly, to such methods which utilize compressional velocity (V.sub.p) and shear velocity (V.sub.s) data.
2. Setting of the Invention
In locating, drilling for, and the production of hydrocarbons from underground formations it is often useful to know certain lithological characteristics of the underground formations. Lithological characteristics include the pore and crack porosity, matrix mineralogy, cementation, permeability, and fluid saturation of an underground formation. Many methods and related apparatus have been used for this purpose with the more common methods including examining core samples from the underground formations, examining the drill cuttings when drilling a wellbore, and utilizing various logging procedures after a wellbore has been established. One disadvantage of these methods is that they all require the drilling of a wellbore, which is an expensive undertaking, or the use of a preexisting wellbore. These measurements also require suitable sample sizes to be recovered, which is often difficult to achieve. Further, permeability cannot be determined from rock fragments.
One method used to determine certain lithological characteristics does not require the use of a wellbore but uses both compressional velocity (V.sub.p) data and shear velocity (V.sub.s) obtained from any commercially available manner, as is well known in the geophysical art. In this method, a plot is made (by hand or with the aid of a computer) of the rate of compressional velocity data/shear velocity data (V.sub.p /V.sub.s) on one axis and either the compressional velocity data (V.sub.p) or the shear velocity data (V.sub.s) on a second axis. It has been found that different formation material types have predictable ranges, i.e., maximum and minimum theoretical values, of V.sub.p and V.sub.s, thus a plot of data points (corresponding to a particular location or depth point in the underground formation) can be made. By using previously obtained data, different areas or boundaries of V.sub.p /V.sub.s or V.sub.p and V.sub.s data can be placed within the plot for each of the desired different formation material types. FIG. 1 represents such a plot and includes typical boundaries for three common formation material types. The data points which fall within a particular boundary can usually be categorized as being of the particular labeled formation material type. This plotting method is described fully in "Basis for Interpretation of V.sub.p /V.sub.s Ratios in Complex Lithologies," by Raymond Eastwood and John P. Castanga, 24th Annual Logging Symposium, SPWLA, June 27-30, 1983, which is herein incorporated by reference.
A major problem with using the previously described plotting method, is that the ratio of V.sub.p /V.sub.s is not monotonically dependent upon porosity regardless of saturation. Specifically, there can exist natural situations where conditions of saturation and crack and pore porosity can cause the ratio to exceed the theoretical maximum value for the uncracked matrix material; this leads to an ambiguous interpretation of the data plotted in such a manner.
There is a need for a method of determining lithological characteristics from compressional and shear data which is easy to interpret and more accurate.
Two methods described in U.S. Pat. Nos. 4,373,197 and 4,375,090 use compressional velocity data and shear velocity data to determine certain mineralogical data; however, these methods are not useful for the determination of lithological characteristics. To differentiate between mineralogical characteristics and lithological characteristics, mineralogy shall be defined as the study of minerals, i.e., naturally occurring homogeneous phases, such as gold, iron, calcite, etc. Lithology shall be defined as the study of combinations of minerals. The methods of these two patents contain no determinant factors for porosity, fluid saturation, or crack porosity. Further, these methods cannot be used to determine the presence of shales in an underground formation because there is no known "single crystal property" of clays, as is specifically needed in these methods.
One of the main lithological characteristics that is desirable to utilize is the underground formation's porosity. Many methods have been used to determine formation porosity, but most of these methods require using a wellbore to obtain core samples, or to operate porosity measuring logging tools. One method used to estimate the porosity of an underground formation without necessarily using a wellbore is to use obtained compressional velocity data in the Wylie Time Average equation, or the Raymer, et al. equation. The Wylie Time Average method of estimating porosity has been found to be accurate for porosities below about 10 percent; however, for porosities above about 10 percent, the porosity prediction using this method is suspect. Another problem in utilizing either of these methods is that a knowledge of the matrix velocity and fluid velocity for the particular geographical area is required. The matrix velocity and the fluid velocity can only be guessed at before the actual formation material is analyzed; therefore, the porosity predictions are dependent upon an operator's "educated guess" or prior experience in a particular geological environment. Also, both of these use only V.sub.p data. Since V.sub.s is much more sensitive to crack porosity and displays a porosity dependence which is different than V.sub.p, it can be used to provide a more accurate measure of total porosity.
There is a need for a method to determine the porosity of an underground formation which uses compressional velocity data and shear velocity data, and which does not require the knowledge of the matrix velocity and the fluid velocity for that particular geographical area.
Another lithological characteristic that is desired to be known is fluid saturation. In reviewing porosity data obtained from commercially available source, it is known to plot the porosity measurement from a neutron logging tool as a function of depth side-by-side with porosity measurements from compensated formation density information. Wherever the two plot lines cross over each other this can be an indication that the formation material at that depth is not fully water or brine saturated, i.e., there can be gaseous hydrocarbons present. This method of indicating the presence of gaseous hydrocarbons is usually accurate; however, it requires the existence of a wellbore to obtain the porosity measurement(s) from a logging tool. There is a need for a method to determine fluid saturation without the need of a well-bore.
In the determination of lithological characteristics, as described above, both compressional velocity data and shear velocity data can be utilized; however, shear velocity data is not always obtainable. One known method to obtain an estimate of shear velocity data is to use compressional velocity data and an estimated Poisson's Ratio. For example, a Poisson solid has a value of .nu.=0.25 and for such materials one can compute V.sub.s from a knowledge of V.sub.p. However, since the deviations from a norm are being sought, i.e., hydrocarbon filled versus brine filled reservoir rock, one cannot presume to know the in situ Poisson's ratio for the brine case without extensive testing or experience. There is a need for a simple method of accurately determining shear velocity data that can be used in the field to determine if shear velocity seismic lines are to be run, and how best to design the field equipment to image the desired objectives.